Stop demo mode detection correctly.
|oobe_timer_| kept running after StopDetection() call. So in fact demo mode
detection remained active.
BUG=439956
TEST=Run 'chromeos=1' debug build with options --derelict-detection-timeout=10 --derelict-idle-timeout=10 --oobe-timer-interval=10. Enroll into enterprise domain, wait for 10 seconds at "Your device has successfully been enrolled." screen. Nothing should happen.
Review URL: https://codereview.chromium.org/
821013002
Cr-Commit-Position: refs/heads/master@{#309418}